Embodiments of the present invention relate generally to electric drive systems including hybrid and electric vehicles and to stationary drives that are subject to transient or pulsed loads and, more particularly, to transferring energy between an electrical storage device of the vehicle or drive and a power source external to the vehicle.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration.
Plug-in electric vehicles, whether of the hybrid electric type or of the purely electric type, are configured to use electrical energy from an external source to recharge the traction battery. Such vehicles may include on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board traction battery. Plug-in vehicles may include circuitry and connections to facilitate the recharging of the traction battery from the utility grid or other external source, for example. The battery charging circuitry, however, may include dedicated components such as boost converters, high-frequency filters, choppers, inductors, and other electrical components, including electrical connectors and contactors, dedicated only to transferring energy between the on-board electrical storage device and the external source. These additional dedicated components add extra cost and weight to the vehicle.
The electricity coming into the vehicle for battery charging can be either direct current (DC) or alternating current (AC). DC charging requires a dedicated charging infrastructure and is considered for fast charging. However, it is easier to utilize existing circuitry when AC charging is employed. According to existing IEC/SAE standards, the AC voltage for charging ranges from 120 volts to 600 volts, single-phase or three-phase. The DC link voltage of a rectifier needs to be higher than the peak of the AC voltage to ensure full controllability of the rectifier to meet power factor and harmonic requirements. In most existing hybrid electric vehicles and electric vehicles, the battery voltage can range from 200 volts to 400 volts. Except for 120 volt inputs, a voltage step down stage is required.
Conventional external charger circuits may include a boost (step up) AC/DC converter or a buck (step down) AC/DC converter. However, these converters have a limited output voltage restricting the ability to control charging the battery. For instance, with 120 volt or 240 volt AC input, the DC output voltage cannot be lower than 170 volt or 340 volt respectively when boost AC/DC converters are used. Furthermore, with 120 volt or 240 volt AC input, the DC output voltage cannot be higher than 170 volt or 340 volt when buck AC/DC converters are used. A DC/DC converter can be included to match the full battery voltage range; however, this leads to increasingly large chargers to compensate for growing power levels, which is ultimately difficult to package into a vehicle.
An example of a conventional external charger circuit is described in U.S. Pat. No. 8,030,884, which is herein incorporated by reference. In the configuration in FIG. 1 of this patent, the output of a diode rectifier is coupled to a DC link of a traction inventor through a plurality of inductors. This configuration is only capable of boosting (not bucking) the output voltage of a diode rectifier determined by an external AC voltage source, limiting charging capabilities. Further, contactors utilized in this configuration need to be sized for operating the current of an electric motor, which can be significantly larger than the charging current, leading to heavier and more expensive contactors.
It would therefore be desirable to provide an apparatus to facilitate the transfer of electrical energy from an external source to the on-board electrical storage device of a plug-in vehicle for all charging conditions that reduces the cost and number of components dedicated only to transferring energy between the on-board electrical storage device and the external source.